The PIs propose to study the neuronal mechanisms underlying extensor spasms in incomplete spinal cord injured (SCI) patients. The responsible neurophysiological mechanisms remain poorly understood. The PIs hypothesize that extensor spasms are due to a state transition in segmental reflex pathways. This state transition has been observed in man during walking and in spinal cats during fictive locomotion. Classical inhibitory reflex actions that are observed under normal conditions switch to facilitatory that drive the extensor spasms and are observable following imposed hip angular variations, as extension of the hip initiates these spasms. Aim 1 is to characterize the neuronal pathways and the contribution of certain types of afferents during imposed hip angular variations under static conditions. Experiments will be performed in which neuronal pathways such as non-reciprocal group I inhibition, disynaptic reciprocal la inhibition, presynaptic inhibition and depression of segmental reflexes following activation of body loading receptors will be estimated during hip imposed angular variations. Aim 2 is to characterize spinal reflex excitability and classical inhibitory pathways at the incipient stage of extensor spasms. Experiments will be performed to investigate at the incipient stages of the extensor spasms the muscle pattern activation, the spinal reflex excitability and the modulation of classical inhibitory neuronal pathways described in man and tightly coupled with motor control. The PIs believe that the above-described experiments will provide significant breakthroughs in rehabilitation of SCI patients. Although, extensor spasms are clinically characterized as a substantial extension of the lower limbs, the neuronal mechanisms underlying these spasms have not been investigated. Therefore, by knowing the neuronal mechanisms, a more effective treatment can be developed thus contributing substantially to a better quality of life. A future study can be concentrated on the potential reflex pathways underlying spasms and their relation to hip extension (end of stance phase) during assisted locomotion in incomplete SCI patients.